1. Field of the Invention
This invention relates generally to oil-in-water emulsions employed as lubricants in metalworking operations, and more particularly to a method of and apparatus for monitoring the emulsion stability and oil concentration and for controlling the addition of oil and emulsifiers to thereby control the concentration and stability of the emulsion.
2. Description of the Prior Art
It is well-known to use oil-in-water emulsions as lubricants in metalworking operations. For example, in operation of rolling mills to shape metal such as aluminum and steel, it is conventional practice to employ an oil-in-water emulsion to flood the work rolls and the metal being shaped as it enters the rolls. In such uses, the emulsion acts both as a lubricant and as a coolant for both the work and the rolls. Such emulsions are also conventionally used in cutting, machining, milling, drawing, grinding and other like metalworking operations where both lubrication and temperature control may be important considerations. The present invention was developed in connection with operation of a steel rolling mill and reference to the operation of such a mill will from time to time be made herein in describing the invention. It is understood, however, that the invention is not limited to such a system, but rather is equally applicable to systems for use in other metalworking operations.
A typical oil-in-water emulsion utilized in a steel rolling operation may comprise an aqueous dispersion of a blend of oils, usually a mineral oil with natural oils such as tallow, along with nonionic and/or anionic emulsifying agents. The emulsion may also contain minor amounts of other ingredients such as bacteriocides and coupling agents which are employed to increase the useful life of the emulsion. A disclosure of such emulsions may be found in numerous prior art publications, such, for example, as U.S. Pat. Nos. 3,783,664 and 3,409,551.
In operation of a rolling mill, an oil-in-water emulsion is conventionally flowed or sprayed onto the work and/or rolls ahead of each set of work rolls. This necessarily results in a loss of a portion of the emulsion due to evaporation, drag-out, spillage, and the like. Variations in temperature of the metal, work rolls and ambient air can produce variations in evaporation rate, particularly of the water component of the emulsion, and various factors including surface conditions of the rolls and the metal being worked affect the loss of water, oil, emulsifiers, and other components. Thus, in order to maintain a consistent oil concentration and emulsion stability, it is necessary to periodically add varying amounts of oil, emulsifiers, and water to the system.
The cooling and lubricating systems employed with rolling mills generally are either of the direct-application type, most frequently employed with specialty orders and short runs, or of the recirculating or reclaiming type. The reclaiming lubrication systems employed with steel rolling mills typically include a large-volume storage chamber, or tank, which may incorporate means for controlling the temperature of the emulsion in the system. Emulsion pumped from this tank to the mill cools and lubricates the workpiece and rolls, then flows downwardly and is collected in a sump beneath the work rolls. The collected emulsion is then purified through suitable filters, screens, and the like before being returned to the storage tank for recirculation. Such systems may have a capacity of many thousands of gallons, and the emulsion in the system may be recirculated and used for several months of substantially continuous operation. However, it is necessary to periodically analyze samples of the emulsion to determine the oil concentration and emulsion stability, as well as other factors such as PH, bacteria count and the like. Depending on the results of the sample analysis, make-up water, oil, emulsifiers and other components are added to control the stability, oil concentration, and other characteristics within prescribed limits.
Direct application systems generally have a substantially lower total emulsion capacity and do not incorporate the sophisticated reclaiming and purifying means employed in recirculating systems. Instead, re-use of the used emulsion is accomplished by pumping the used emulsion from the sump to a holding and mixing tank where it is continuously agitated or mixed to prevent separation and from which it is pumped back to the point of application. Again, however, the composition and stability of the emulsion must be maintained within prescribed limits and this is accomplished by the periodic addition of oil, water, emulsifiers, and the like. As a general rule, the emulsion in a direct application system is discarded after use for a comparitively short time such as after a specified number of turns or after completion of a particular order or job.
In the past, it has been conventional practice to periodically draw emulsion samples for laboratory-testing to determine oil concentration and emulsion stability. The concentration has generally been determined by centrifuging or by salt-split testing, both procedures being well-known in the art. The stability of the emulsion is represented by an emulsion stability index (ESI), which may be determined by conventional test procedures such as those contained in ASTM 3342-74, and which is an indication of the rate of separation of the oil and water phases in a quiescent emulsion.
It has also been proposed to flow a portion of an emulsion through a continuous centrifuge to separate the oil and water components which are then caused to flow through separate flow meters which generate signals proportional to the flow of the two components. A comparison of the two signals is employed to periodically add slugs of oil, as required, to maintain the proper concentration of oil in the emulsion. Such a system is disclosed in U.S. Pat. No. 3,153,420.
U.S. Pat. No. 3,954,119 discloses a system for controlling the oil concentration in an emulsion by measuring the rate of propagation of ultrasonic vibrations through the emulsion flowing in a pipe and relating this measured rate to the emulsion density, and therby emulsion concentration. The propagation rate is utilized to control the addition of oil and water to maintain the proper oil concentration in the emulsion.